I am using ArcMap 10.2 and merging several line segments with the same ID. Most of the time this works fine and I end up with one segment. (about 95% of the time)

However, occasionally the segments will merge but I end up with two separate "parts". The "parts" I am speaking of are what you can see when you "edit vertices" and look to the "Edit Sketch Properties". It says "Part 0" ....lists all vertices..."Part 1"...lists all vertices

The end goal is building "M" values for these merged segments and I haven't found a workaround other than splitting the merged segment before the second "part" starts and extend/tracing over the original part that wouldn't merge properly. This is becoming very tedious with larger segments.

When you get a multi-part feature it means the two parts don't share common geometry. As Maksim suggests, Dissolving on the ID field and not allowing multiparts will take care of most of the work you're doing in a single step. However in the cases like you describe, you'll have to manually edit the lines to fix the breaks - somehow one endpoint isn't snapped to another. You don't need to retrace the entire line, just edit the last vertex. This may also be arising from geometry errors (line goes out then comes back on itself so the 'end' is in the 'middle'). Geometry Check may be in order.
– Chris WDec 2 '14 at 19:55

If you already have multi-parts, there's the Explode and Multi-parts to Single-parts tools as opposed to manual splitting. They can have different effects on feature attributes.
– Chris WDec 2 '14 at 19:58

1 Answer
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Using the Dissolve (Data Management) tool on your ID field for your lines, you can specify whether the resultant lines will be multi-part or single-part via the multi-part parameter. You can also dissolve or unsplit lines via the unsplit parameter.